Method for providing improved nucleate boiling surfaces

ABSTRACT

An improved nucleate boiling surface can be obtained on a metal heat transfer tube or other form of heat transfer member by knurling or otherwise deforming a smooth surfaced member to notch and partially work harden its surface and then subjecting the knurled portion to a finning operation. The method results in small splits or ruptures of a controlled geometry and depth being produced in the tip area of the fins which become nucleation sites for boiling enhancement. Preferably, the knurling rolls are mounted on the same arbors as the finning disks so that a tube is progressively knurled and then finned in a single pass through the apparatus. If desired, the tube can also be internally ridged as it is being externally finned.

United States Patent Campbell et al.

METHOD FOR PROVIDING IMPROVED NUCLEATE BOILING SURFACES Inventors: Bonnie J. Campbell; Klaus K.

Rieger, both of Decatur, Ala.

Universal Oil Products Company, Des Plaines, lll.

Filed: Aug. 21, 1974 Appl. No.: 499,344

Assignee:

US. Cl 72/98; l65/133 Int. Cl B2lh 3/12 Field of Search 72/98, 100; 29/1573 AH;

Reierences Cited UNITED STATES PATENTS 3/l939 Berger 72/l00 6/1967 Ware 165/184 8/l97l Klug et al. 72/98 July 8, 1975 Primary Examiner--Lowell A. Larson Attorney, Agent, or Firm-James R, Hoatson, Jr.; Barry L. Clark; William H. Page, ll

[ 57] ABSTRACT An improved nucleate boiling surface can be obtained on a metal heat transfer tube or other form of heat transfer member by knurling or otherwise deforming a smooth surfaced member to notch and partially work harden its surface and then subjecting the knurled portion to a timing operation. The method results in small splits or ruptures of a controlled geometry and depth being produced in the tip area of the fins which become nucleation sites for boiling enhancement. Preferably, the knurling rolls are mounted on the same arbors as the finning disks so that a tube is progressively knurled and then finned in a single pass through the apparatus. If desired, the tube can also be internally ridged as it is being externally finned.

5 Claims, 4 Drawing Figures PATENTEDJUL 8 1915 3893.322

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a mw/RL 0 F/NNED 42 1 g AREA 0 :311.

I TUBE DIRECTION OF FE 1 METHOD FOR PROVIDING IMPROVED NUCLEATE BOILING SURFACES BACKGROUND OF THE INVENTION This invention relates to the improvement of the efficiency of heat transfer of a metal tube by enhancing its surface. It is known in the art that modifying the surface of a plain cylindrical tube such as by finning or corrugating it or by scoring, knurling, or roughening its surface will increase its heat transfer capability in boiling of liquids substantially as compared to a plain tube. US. Pat. No. 3,454,081 teaches beneficiation ofa planar heat transfer surface via a scoring and knurling technique in which the ridges formed by scoring are partly deformed, by a subsequent knurling operation, into the grooves separating them to obtain partially enclosed and connected subsurface cavities for vapor entrapment and the consequential promotion of nucleate boiling. US. Pat. Nos. 3,326,283 and 3,602,027 teach knurling after finning while US. Pat. Nos. 3,683,656 and 3,696,861 teach partially bending over the fins to form cavities. U.S. Pat. No. 3,789,915 teaches that a tube may be corrugated or knurled (but not both) to get a folded metal configuration having subsurface cavities. US. Pat. No. 3,355,788 relates to heat transfer of gas rather than boiling liquids and discloses the use of saws to slit finned tubing to increase turbulence. As a general rule, finning has always been done on plain smooth surfaces because it is known that surface imperfections can lead to fin splits of uncontrolled depth and orientation. This type of fin imperfection has been avoided in the past for fear that splits penetrating into the tube wall might cause undue degradation of mechanical properties. Splits are now identified non destructively by eddy current test methods, which are applied at the tube manufacturing stage. Generally, splits are not permitted to penetrate deeper than of the tube wall thickness a tube having a split in excess of 10% is scrapped. However, splits in the fin tips and those not penetrating deeper than 10% of the tube wall thickness are acceptable.

Since heat transfer tubing is generally made of expensive materials such as copper and is used in large quantities, it is obvious that improvements in heat transfer efficiency and/or in manufacturing costs can be quite significant in reducing the overall cost of a given heat exchange installation.

It is an object of this invention to provide an improved process for increasing the heat transfer capability of finned tubing in nucleate boiling applications.

It is another object of this invention to provide an improved process for increasing the nucleate boiling heat transfer capability of finned tubing which does not require additional or secondary metalworking operations on the tube.

SUMMARY OF THE INVENTION The aforementioned objects are attained by the improved process of the present invention in which, in a preferred embodiment, a plain surface tube is first knurled at an angle to its axis so that its surface is lightly impressed or embossed with a diamond pattern of grooves having rounded bottoms and is then subjected to a finning operation. The knurling operation tends to work harden the tube in the region of the knurling impression and the additional metalworking inherent inthe finning operation then causes the impressions to split or rupture, thereby forming a great number of very small splits or cavities in the outer radial portions of the fins which are nucleation sites for boiling enhancement. The diamond knurling pattern is preferably obtained by using a pair of knurling tools having their ridges arranged at an angle to each other and at about a 30 angle to the tube axis. The resultant diagonally oriented splits which are promulgated in the fin tips average approximately 0.003 inch in width but are tapered over their depth of about 0.025 inch so as to vary in width from about 0-0.006 inch. The splits are much narrower than could be produced by a slotting operation and are suff ciently narrow as to be capable of initiating and sustaining nucleate boiling. Since the splits are confined to the fin tips, they do not weaken the fins or the tube as they could if they penetrated to a greater depth into the tube wall. Although a pair of knurling tools are preferable for use in deforming a tube surface, other types of tools could also be used which would provide a local workhardened condition. Furthermore, a knurling tool producing grooves aligned with the tube axis could be used but would form shorter splits than an angled groove.

Use of our improved method in the manufacture of finned .tubing has been shown in preliminary tests to provide an improvement in heat transfer efficiency on the outside surface of a finned tube whereby the outside film coefficient of heat transfer was increased approximately as compared with the unimproved reference finned tube. Since the overall heat transfer efficiency of a tube is a function of both the inside and outside heat transfer coefficients, it is desirable to enhance the inside coefficient, such as by internal ridging, in order to take maximum advantage of the external heat transfer improvement offered by the invention. However, even with a plain inside surface the overall improvement for the tube appears to be about 25%.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side view of a portion of an improved heat transfer tube made in accordance with the invention;

FIG. 2 is a fragmentary sectional view of a portion of the tube shown inE FIG. 1 taken on line 2-2 of FIG. 1',

FIG. 3 is a perspective view of a suitable work station (with one arbor removed for clarity) for producing the tube of FIG. 1; and

FIG. 4 is a side view of one of the knurling and finning tools shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is illustrative of a tube indicated generally at 10 made by the improved process of the present invention. The tube 10 is preferably made with plain unfinned portions 12 on each of its ends to facilitate connection of the tube to tubesheets or fittings. The tube may be made with the apparatus 13 shown in FIG. 3 by feeding it from left to right over a mandrel 14 until the portion 12 which is to be left plain has passed the final finning discs 16. At this point, the skewed axis rotating tool arbors 18,20 and 22 are gradually moved into full depth contact of the knurling tools 26,28, the initial finning discs 30 and the final finning discs 16 with the tube 10 by virtue of the fact that the arbors are pivoted on cam arms 32 for movement toward and away from the mandrel 14. Since the tube moves to the right as it is 3 engaged by the various tools, it is obvious that it will initially be knurled in the diamond pattern produced by the opposed diagonal knurling ridges on knurls 26,28 before it is finned. The portions which have been knurled will then immediately beinitially and finally finned by tinning discs 30 and 16 respectively. As can be seen in more detail in FIG. 4, the knurling and finning too'ls 26,30 and 16 are generally tapered to permit them to gradually deform'the advancing tube.

FIG. 2' illustratesthetype of fin splitting produced by our improved method of knurling a tube before it is finned. Since the splits 36 have a tapered width in a radially inward direction they will provide a variable width groove which permits vapor bubbles to form at the most favorable depth for the type of boiling liquid being used. Generally, the more fins that are used and the greaterthe number of splits 36 that are present, the more the-heat transfer capability of the tube will be increased in boiling. We have found that a standard No. 265028 tube having 26 fins per inch, a five-eighths inch root diameter and a wall thickness under the tins of 0.028 in. which was knurled before finning to a depth of about 0.005-0.007 inches with tools 26,28 having a tooth pitch of 0.030 inches provided an improvement in outside surface heat transfer coefficient in boiling in refrigerent R42 of about 80% as compared to a tube having identical fins but without the preknurling feature.

Weclaim as our invention:

1. A process for improving the heat transfer capability of a metal heat transfer surface comprising the steps of:

Knurling at least a portion of said heat transfer surface and then subjecting said knurled portion to a finning operation.

2. A process in accordance with claim I wherein said knurling step produces a controlled pattern of work hardened regions on the surface which, when subjected to additional metal working by the finning operation, result in splits of controlled depth and orientation in the outer portions of the fins, said splits being sufficiently small to serve as nucleate boiling sites when said heat exchange surface is in contact with a boiling liquid.

3. A process in accordance with claim 2 wherein said heat transfer surface is on a tube.

4. A process in accordance with claim 3 wherein the controlled pattern of work hardened regions comprises a pattern of grooves produced by said knurling step which are diagonally arranged relative to the tube axis and generally transverse to the plane of each fin.

S. A process in accordance with claim 1 wherein said tube is subjected to said knurling and timing operations simultaneously. 

1. A process for improving the heat transfer capability of a metal heat transfer surface comprising the steps of: Knurling at least a portion of said heat transfer surface and then subjecting said knurled portion to a finning operation.
 2. A process in accordance with claim 1 wherein said knurling step produces a controlled pattern of work hardened regions on the surface which, when subjected to additional metal working by the finning operation, result in splits of controlled depth and orientation in the outer portions of the fins, said splits being sufficiently small to serve as nucleate boiling sites when said heat exchange surface is in contact with a boiling liquid.
 3. A process in accordance with claim 2 wherein said heat transfer surface is on a tube.
 4. A process in accordance with claim 3 wherein the controlled pattern of work hardened regions comprises a pattern of grooves produced by said knurling step which are diagonally arranged relative to the tube axis and generally transverse to the plane of each fin.
 5. A process in accordance with claim 1 wherein said tube is subjected to said knurling and finning operations simultaneously. 